The present invention relates to a continuous process for the high-pressure polymerization of 1,1-difluoroethylene (VF2) to give polyvinylidene fluoride (PVDF).
The polymerization of 1,1-difluoroethylene is currently carried out industrially in an aqueous medium either in emulsion or in suspension. This type of polymerization thus corresponds to batchwise processes.
In the case of the emulsion, it is necessary to add emulsifiers to the polymerization medium in order to stabilize the PVDF latex particles formed. The emulsifiers must be removed in a subsequent step in order to ensure that the polymer is in a suitable purity. Furthermore, certain emulsifiers which are used in the emulsion process are perfluoro molecules. Studies have shown that these molecules have a tendency to accumulate in aquatic flora. Although considered as harmless, nothing is as yet known regarding the long-term impact of this bioaccumulation.
In the case of the suspension, protective colloids are incorporated in order to stabilize the PVDF particles.
Another process is currently under investigation in many laboratories. This continuous process differs from emulsion/suspension processes in that it is carried out in supercritical CO2 (ScCO2) without stabilizing additive. During the polymerization, the PVDF which is insoluble in the ScCO2 precipitates and forms a powder. However, it is necessary to add a powder-treatment step in order to convert it into PVDF granules since this type of process does not make it possible to obtain a powder of controlled and narrow particle size (as with the emulsion process).
Patent FR-A-1 260 852 discloses the polymerization of VF2 under pressure either in the presence of a neutral reaction medium such as water or in the absence of a reaction medium. According to the first form, deionized and deoxygenated water are loaded into an autoclave, followed by a peroxide and VF2, the proportion of peroxide being 0.8 g per 35 g of VF2. The autoclave is closed and heated, the pressure establishes at values of about 40 to 60 bar, and the reaction time is about 18 hours. Next, the autoclave is cooled and PVDF is recovered therefrom. According to the second form, the process is performed as in the first form, but without introducing water, the pressure and the reaction time being the same. It is stated that this second form lends itself to a continuous operation in which the autoclave is connected to a source of VF2 under pressure such that the fresh VF2 enters the autoclave when the conversion into polymer takes place. According to a variant, catalyst may be added continuously or in batchwise mode. This prior art is based on a fatal pressure (autogenous pressure) generated by the volume of the autoclave, the reagents loaded in and the temperature. The fact that it can be made continuous by injecting fresh VF2 to compensate for the VF2 which is polymerized has nothing to do with a process in which the reaction pressure is determined by the pressure supplied by the VF2 injection pumps. Nothing is stated regarding the oxygen content of the VF2.
Patent U.S. Pat. No. 2,435,537 discloses a process similar to the previous one but still in the presence of water, 50 parts per 40 parts of VF2, and the pressure may be 1000 bar. All the examples are in batchwise mode in an autoclave. It is stated that the process may be performed continuously; however, it is recommended always to use an inert medium such as water to disperse the catalyst and control the reaction by dissipating the heat. It is stated that oxygen has a harmful effect on the polymerization, but the oxygen content of VF2 is not specified and nothing is stated regarding the means for reducing it.
Patents FR-A-2 650 593 and FR-A-2 689 134 disclose processes for the high-pressure synthesis either of copolymers of VF2 and of ethylene or copolymers of VF2 and of fluoroacrylates.
Patent WO 98/28351 discloses the continuous polymerization of VF2 in supercritical CO2 (75xc2x0 C.xe2x80x94276 bar). PVDF copolymers may thus be manufactured. In one example, the flow rate of fluoro monomers is 200 g/h for a CO2 flow rate of 518 g/h.
A high-pressure process exists, in which the VF2 is converted into PVDF solely under the effect of pressure and traces of a peroxide. The high-pressure polymerization makes it possible to overcome the problems mentioned in the other processes cited above:
the process does not require stabilizing additives (protective colloids or surfactants),
no treatment of the powders.
Another advantage of the high-pressure process is that it offers better production efficiencies than the continuous process in ScCO2.
The present invention relates to a process of this type. Particularly, a continuous process for preparing PVDF homopolymer or copolymer has now been found, the comonomer being a fluoro monomer rather than an acrylate, in which no organic solvent or water is used and which is not in ScCO2 medium.
The invention is a continuous process for the manufacture of PVDF homopolymer or copolymer, the comonomer being chosen from compounds containing a vinyl group capable of being opened by the action of free radicals in order to polymerize, and which contains, directly attached to this vinyl group, at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxy group in which:
(a) a flow of VF2, of optional comonomer and of radical initiator is introduced into a reactor maintained at a pressure of between 300 bar and 3000 bar, the reactor containing essentially VF2, an optional comonomer and PVDF;
(b) a flow of reaction mixture is removed from the reactor for step (a) and introduced into a separator;
(c) molten PVDF is recovered in the separator and purged continuously;
(c1) the flow of PVDF from step (c) is optionally introduced into a device to place it in the form of granules;
(d) VF2 and optionally comonomer are recovered in the separator and recycled into step (a).
According to one advantageous form of the invention, the fresh VF2, the other portion being recycled, which is introduced into step (a) contains less than 5 ppm of oxygen, preferably less than 1 ppm and better still between 0.1 and 0.8 ppm.
The process of the invention advantageously comprises an additional step consisting in deoxygenating the fresh VF2 before introducing it into step (a).
This step consists in placing the flow of VF2 in contact with (i) a sufficient amount of a catalyst whose active sites are elements belonging to groups 8 to 11 of the Periodic Table of the Elements and (ii) for a time which is sufficient to obtain the desired oxygen content.
The invention also relates to this isolated step of treatment of VF2 to reduce its oxygen content. This step advantageously precedes a process which is different from the preceding one in which the VF2 is polymerized or copolymerized.
This deoxygenation may also apply to mixtures of VF2 and of one or more comonomers and also to the VF2 comonomers alone.
The invention also relates to, as a product, VF2, the mixture of VF2 and of comonomer or the comonomer alone (or mixture of comonomers) containing less than 5 ppm of oxygen, advantageously less than 1 ppm and better still between 0.1 and 0.8 ppm.
The present invention also relates to a PVDF with an Mw/Mn ratio of between 1.5 and 1.9.
The process of the invention has many advantages:
there is no water or organic solvent: it is a clean process, not requiring the recycling/treatment of water or the recycling of solvent;
it is a  less than  less than dry  greater than  greater than  process not requiring the removal of water or solvent from the polymer: it is less expensive in energy terms;
the polymer is cleaner since it is not soiled by the presence of surfactants or other additives;
there is no use of perfluoro surfactants which may bioaccumulate in the environment: the process is environmentally friendly;
there is no handling of powder as in the ScCO2 process: this entails a simplification of the process, with no cyclones to be added, no step of uptake of the powder to convert it into granules, no problem of powder explosion and no problem of electrostatics;
the production efficiencies are better than in the ScCO2 process (as demonstrated in Example 1).
As regards the proportions of VF2 and of comonomer in the PVDF and thus the proportions of fresh VF2 and of fresh comonomer introduced into step (a), the proportion by weight of comonomer is advantageously between 0 and 50% and preferably between 0 and 30%.
Examples of comonomers which may be mentioned are vinyl fluoride; trifluoroethylene; chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propyl vinyl) ether (PPVE); perfluoro(1,3-dioxole); perfluoro(2,2-dimethyl-1,3-dioxole) (PDD); the product of formula CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2X in which X is SO2F, CO2H, CH2OH, CH2OCN or CH2OPO3H; the product of formula CF2xe2x95x90CFOCF2CF2SO2F; the product of formula F(CF2)nCH2OCFxe2x95x90CF2 in which n is 1, 2, 3, 4 or 5; the product of formula R1CH2OCFxe2x95x90CF2 in which R1 is hydrogen or F(CF2)z and z is 1, 2, 3 or 4; the product of formula R3OCFxe2x95x90CH2 in which R3 is F(CF2)zxe2x80x94 and z is 1, 2, 3 or 4; perfluorobutylethylene (PFBE); 3,3,3-trifluoropropene and 2-trifluoromethyl-3,3,3-trifluoro-1-propene. Several comonomers may be used.
The reactor contains at least 90% by weight of VF2, one (or more) optional comonomer and PVDF. There is no organic solvent, water or ScCO2.
The reactor pressure is advantageously between 500 and 3000 bar and preferably between 1500 and 2500 bar. The reactor has a volume such that the residence time is advantageously between 1 min and 1 h.
As regards the radical initiator, the product is known per se. Suitable radical initiators which may be used comprise tert-butyl perpivalate, t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, 1,3-bis(t-butylperoxyisopropyl)benzene, acetyl peroxide, benzoyl peroxide, isobutyryl peroxide, bis(3,5,5-trimethyl)hexanoyl peroxide and methyl ethyl ketone peroxide. The initiator may be dissolved in a solvent; examples which may be mentioned are heptane and isododecane.
The flow rate of initiator is advantageously between 2 ppm and 1000 ppm by weight relative to the flow rate of fresh VF2 or of the fresh VF2 and fresh comonomer together, and preferably between 2 ppm and 100 ppm.
The flow rate of VF2 and of optional comonomer is adjusted to compensate for the production of PVDF, and the temperature is adjusted by the reactor-cooling system. The flow rate of initiator is adjusted to obtain a conversion of the monomers into PVDF. The temperature is advantageously maintained between 50xc2x0 C. and 300xc2x0 C. and preferably between 90xc2x0 C. and 220xc2x0 C. The advantage of working at a temperature above 150xc2x0 C. to obtain low crystallinity is explained later.
The apparatus used can be the same as that in which the synthesis of low-density polyethylene (LDPE) is carried out. This apparatus is known.
The autoclave and tubular processes both form part of the so-called xe2x80x9chigh-pressurexe2x80x9d polymerization processes and a person skilled in the art immediately knows what it involves. These two processes involve the high-pressure radical-mediated polymerization of ethylene, at pressures generally of between 100 MPa and 350 MPa and at temperatures above the melting point of the polyethylene being formed. The tubular process involves the polymerization in a tubular reactor. A tubular reactor comprises cylinders whose inside diameter is generally between 1 cm and 10 cm and whose length is generally from 0.1 km to 3 km. In a tubular reactor, the reaction medium is driven at high linear speed, generally of greater than 2 meters per second and short reaction times, which may be, for example, between 0.1 min and 5 min.
The pressure in a tubular reactor may be, for example, between 200 MPa and 350 MPa.
The autoclave process involves polymerization in an autoclave whose length/diameter ratio generally ranges from 1 to 25 in the case of a single-zone reactor. In the case of a multiple-zone reactor, the ratio of the length of each zone to the diameter generally ranges from 0.5 to 6, it being understood that the reaction medium flows in the direction of the length. The pressure in an autoclave reactor may be, for example, between 100 MPa and 250 MPa.
It would not constitute a departure from the context of the invention to add to the reactor or at the outlet a product for promoting flow. By way of example, this product may be supercritical CO2, but this has nothing to do with a polymerization in ScCO2. Specifically, the amount added is from about 0 to 10% by weight relative to the mass of fluoro monomers and of PVDF contained in the reactor.
As regards the oxygen content of the VF2, it is generally known that the presence of oxygen may have an appreciable influence on radical-mediated polymerizations, whether they are carried out in emulsion, suspension or bulk, at low pressure (P less than 500 bar) or at high pressure (P greater than 500 bar). Specifically, the oxygen participates in the radical-mediated processes and can act either as initiator or as polymerization inhibitor. Reference may be made to Chemical Reviews 1991, 91 (2), 99-117 for further details regarding the exact mechanisms involving the O2 species. However, it was not known that VF2 contained amounts of oxygen which could impair its polymerization, whichever process is used.
For example, when VF2 is manufactured by a process of cracking 1-chloro-1,1-difluoroethane:
CH3xe2x80x94CF2Clxe2x86x92HCl+VF2
This process does not make it possible to obtain VF2 containing less than 5 ppm of residual oxygen. Generally, the VF2 obtained from this process contains between 10 ppm and 15 ppm of oxygen, which is much too high for polymerization, in particular at high pressure. It is thus desirable to have available a simple and economical single operation for removing the residual oxygen in the VF2 and possibly in the comonomers.
The catalyst used is a solid catalyst composed of active sites dispersed on a mineral support, optionally containing promoters whose role is to increase the chemical activity of the catalyst. The active sites are elements belonging to groups 8-11 of the Periodic Table.
The solid catalyst is in the form of granules of varied shapes (cylinders, flakes, beads, etc.). The support for the active sites is preferably mineral. It may be alumina, silica, zeolite or aluminosilicate or any other support known to those skilled in the art of heterogeneous catalysis. The catalyst contains between 0.05% and 5% by weight of the active element.
In order to ensure optimum catalytic activity, the support is preferably porous. The catalyst has a pore volume of between 0.001 ml/g and 1000 ml/g, preferably between 0.01 ml/g and 100 ml/g. The specific surface area of the catalyst makes it possible to assess the catalyst""s capacity to expose its active sites to the flow of reagents. The specific surface area is expressed in m2/g; the active surface area is between 1 m2/g and 10,000 m2/g, preferably between 1 m2/g and 1000 m2/g.
A magnitude, known as the space velocity, may be defined, which relates the flow rate of gas to the amount of solid catalyst. This magnitude may be expressed in Nm3/m3/h or in kg/kg catalyst/h, and is between 0.01 kg/kg/h and 1000 kg/kg/h, preferably between 0.1 kg/kg/h and 100 kg/kg/h.
The process is useful for reducing the oxygen content from 100 ppm to less than 5 ppm and advantageously to less than 1 ppm and more particularly from 30 ppm down to 0.2 ppm or 0.8 ppm.
The process may be performed at any pressure, which has no effect on such small contents. The advantage of a high pressure is that the apparatus for placing the VF2 in contact with the catalyst is more compact. The pressure at which the VF2 to be deoxygenated is available is usually used.
As regards the temperature, it may be between 0xc2x0 C. and 200xc2x0 C. However, it is advantageously between 50xc2x0 C. and 100xc2x0 C.
Preferentially, the element is copper. When the element is copper, the removal of oxygen is based on the following chemical equations:
2Cu+O2xe2x86x922CuO
4Cu+2O2xe2x86x92Cu2O
The removal of oxygen is carried out by simply passing the monomer(s) through a cartridge. The catalyst contained in the cartridge is preferably a solid catalyst containing the elements copper or palladium. On the industrial scale, given the larger flow rates by volume of monomer which it is necessary to treat, the cartridge is replaced with apparatus of larger volume. This may be, for example, a column or an assembly of two or more columns functioning in series or in parallel. If the columns function in parallel, a step of catalyst regeneration may take place while another column carries out the deoxygenation of the monomer.
Another solution for deoxygenating the monomer consists in combining with the VF2 container a deoxygenating cartridge directly linked to the container. The deoxygenation thus takes place in semi-continuous mode each time the monomer is removed from the container.
This type of catalyst is already known and is used to treat the neutral gases (argon, nitrogen) of laboratory glove boxes. This is necessary, for example, for the handling of organometallic compounds which are oxygen-sensitive (for example organometallic aluminum derivatives). These catalysts are also used to remove the traces of oxygen from gaseous monomers containing only carbon and hydrogen, for example ethylene or propylene. Specifically, in order to polymerize ethylene efficiently using catalysts of Ziegler-Nafta type, it is necessary to have available monomers containing minute traces of oxygen.
The catalyst of the type R3-15 T5x3 sold by BASF(copyright) may be mentioned for example.
This deoxygenation is useful in the high-pressure PVDF preparation process described above, but also in the other PVDF preparation processes. Case of the high-pressure polymerization according to the main process of the present invention:
The bulk polymerization of 1,1-difluoroethylene to give PVDF is a polymerization carried out at high pressure, and requires the removal of all traces of oxygen from the fluoro monomer in order to avoid an untimely polymerization in the reactor or even in the pumps during the compression phase. Specifically, the oxygen may act as radical initiator in the same way as the organic radical initiators which are intentionally added to the polymerization medium. The presence of oxygen thus impairs the working of the process and also its safety.
For example, when the bulk polymerization of VF2 is carried out at 1850 bar, it is necessary to inject the equivalent of 7 ppm of pure tert-butyl perpivalate (sold under the reference code LUP 11 by Atofina). However, if the monomer contains 10 ppm of residual oxygen, this is equivalent to an amount of LUP 11 of 54 ppm, i.e. much more than is required to carry out the polymerization. If the oxygen content is lowered to 1 ppm of residual oxygen, the equivalent amount of LUP 11 is 5.4 ppm.
Case of the Emulsion Polymerization
In the case of the emulsion polymerization of VF2, an excessive amount of oxygen may retard the polymerization or even prevent it from taking place. Tests have shown that at and above an oxygen content of 20 ppm, the polymerization is totally inhibited.
When the polymerization of VF2 takes place in emulsion with initiation with ammonium persulphate, chain ends are terminated with fragments of the initiator. These ends are fragile and on heating give sulphuric acid which degrades the polymer. Similarly, the use of percarbonates as initiators leads to the formation of aldehydes as decomposition by-products. These aldehydes may give rise to colorations of the polymer when this polymer is extruded or converted at high temperature. Consequently, PVDF prepared in emulsion is proportionately more stable the lower the amount of initiator required. There is thus a need to reduce the residual oxygen content in order to use as little initiator as possible.
For example, when the emulsion polymerization of VF2 is carried out, 1 g of potassium persulphate per 10 kg of PVDF is frequently used, which is equivalent to 7.7xc3x9710xe2x88x927 mol of radical anions per gram of PVDF. Now, if the monomer contains 10 ppm of residual oxygen, this is equivalent to a theoretical amount of 4.3xc3x9710xe2x88x927 mol of Oxc2x0 radicals per gram of PVDF (for a final solids content of about 40%). Without prejudging the inhibition mechanism, it is found that the concentrations are of the same order of magnitude. The potential advantage of having a VF2 containing an amount of residual oxygen of less than 1 ppm, especially for the VF2 added during the polymerization and which does not undergo the initial degassing procedure, may thus be appreciated.
As regards PVDF of high melt flow index, another aspect of the invention relates to the possibility of obtaining PVDFs (or corresponding copolymers) of high melt flow index (MFI). Specifically, it is known that there is a limit to the production of PVDF of high melt flow index by emulsion or suspension processes.
The process disclosed in the present invention is particularly flexible as regards the production of these products of high melt flow index. The expression xe2x80x9cmelt flow indexxe2x80x9d means the mass of resin flowing through a die in a given time, at a given temperature and under a given weight. It is thus a fully standardized measurement. In the case of PVDF or its copolymers, the following standard is applied: the measurement is carried out at 230xc2x0 C., under a 5 kg load, through a die 2.09 mm in diameter. PVDFs with an MFI value of greater than 50, 100, 200 or even 400 g/10 min (at 230xc2x0 C./5 kg according to ASTM D-1238) are prepared.
The molecular masses are controlled by injecting transfer agents into the reactor. The same transfer agents as for the production of polyethylene may be used. Highly efficient transfer agents exist, which are well known in the processes for producing low-density polyethylene. They are molecules containing labile hydrogen atoms. Mention may be made, for example, of alcohols, aldehydes, in particular propanal or butanal, or even alkanes or alkenes, for example butane, propylene, heptane or isododecane.
These PVDFs of high melt flow index are useful for making coatings. The present invention also relates to, as products, these PVDFs having these MFIs.
As regards PVDF of low crystallinity, the emulsion or suspension processes are carried out in the presence of water, which necessarily limits the polymerization limit temperature. This is generally between 80xc2x0 C. and 100xc2x0 C. It is an advantage to be able to work at high pressure, without water, since it is easy to polymerize VF2 at temperatures above 150xc2x0 C. At these temperatures, the number of defects present in the polymer chains increases, which has an effect on the crystallinity of the polymer and thus also on its melting point. The larger the number of defects, the more the crystallinity decreases and the more the melting point decreases.
The expression xe2x80x9cinversion defectxe2x80x9d means any combination of the type xe2x80x94CH2xe2x80x94CF2xe2x80x94CF2xe2x80x94CH2xe2x80x94 along the polymer chain (this also being known as the head-head combination as opposed to the head-tail combination of the type xe2x80x94CH2xe2x80x94CF2xe2x80x94CH2xe2x80x94CF2xe2x80x94). The number of defects may be measured by the fluorine NMR. The number of inversion defects is thus generally given as a percentage.
The level of inversion defects may be between 5% and 15% and for example greater than 6 or 7%. The present invention also concerns, as products, these PVDF homopolymers having these levels of inversion defects.
Another way of measuring the crystallinity of the PVDF is to measure the melting point. It is thus an advantage to be able easily to obtain PVDFs with a melting point of less than 162xc2x0 C. and advantageously between 162 and 135xc2x0 C.
This temperature may be for example 155xc2x0 C., indeed less than 150 or 145xc2x0 C. The present invention also concerns, as products, these PVDFs with these melting points.
Another way of measuring the crystallinity of the PVDF is to measure the elastic modulus. In an entirely advantageous manner, it has been noted that the PVDFs prepared under high pressure had low moduli, giving rise to the possibility of using these products in Kynar Flex(copyright) applications (that is to say VF2-HFP copolymers). The elastic modulus at 23xc2x0 C. according to ASTM D-1708 may be of between 1020 and 650 MPa. The modulus may be for example less than 1000, 900, 800 or even 700 MPa. The present invention also concerns, as products, these PVDF homopolymers with these elastic moduli.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all cited applications, patents and publications, and of corresponding French application 00/12191, filed Sep. 26, 2000, is hereby incorporated by reference.